Our long-term objectives are to clarify the role of bombesin-like peptides (BLPs) from pulmonary neuroendocrine cells (PNECs) in primate lung development and to determine their clinical usefulness in treating premature infants to prevent respiratory distress syndrome (RDS) and bronchopulmonary dysplasia (BPD). BLPs are likely to participate in normal mammalian lung development because high BLP levels are found in fetal lung. We previously demonstrated transient expression of mammalian BLP mRNAs in mid-gestation human fetal lung in a proximal-to distal fashion in parallel with growth of the airways. In utero BLP administration accelerated fetal lung growth and maturation; endogenous lung maturation was blocked by an anti-BLP monoclonal antibody. the cell surface enzyme CE10/neutral endopeptidase 24.11 (CD10/NEP) has been found to hydrolyze and inactivate BLPs, which are mitogens for normal bronchial epithelial cells, pulmonary fibroblasts, and many small cell carcinomas of the lung. Inhibition of CD10/NEP by SCH32615 potentiated human fetal lung growth in organ cultures and both growth and maturation in murine fetal lung in utero. All of these effects were blocked by BLP receptor antagonists. It is possible that treatment with exogenous BLPs or potentiation of endogenous BLPs by CD10/NEP inhibition might prevent BPD. The baboon is the only available animal model to study this important human disorder. Our First Aim is to characterize PNEC ontogeny and the expression of BLP, BlP receptor, and CD10/NEP genes in developing normal fetal baboon lung and in animals with hyaline membrane disease (HMD) and BPD. Second, we will determine he optimal combination of BLPs with dexamethasone (DEX) and/or thyroid hormone (T4) for primate fetal lung growth and maturation in organ cultures as a prelude to in vivo studies. Growth will be assessed using: 3H-thymidine incorporation into nuclear DNA; PCNA immunostaining; and histopathology. Maturation will be assessed using: 3H-choline incorporation into surfactant phospholipids; histopathology and electron microscopy; immunostaining for markers of differentiated type II cells and PNECs; and quantitation of relative mRNA levels for surfactant phospholipid synthesis. Third, we will investigate in vivo effects of the optimal combination(s) of BLPs with other agents on baboon fetal lung growth and maturation at 140 days gestation, which will be assessed as described above. Fourth, we will determine whether the best treatment regimen shown to enhance lung maturation without growth inhibition under Aim 3 improves the clinical outcome of fetal baboons delivered at 140 days gestation, with our major focus on BPD. Finally, we will assess alterations in PNECs and expression of BLP, BLP receptor, and CE10/NEP genes in developing lung following in vivo treatment with Dex plus T4, or with retinoic acid (RA) in normal baboons and in animals with HMD and BPD. This investigation would be a critical prelude to the application of these agents in clinical medicine.